The view you’re seeing here is the stack’s collapsed view (where
11 similar photos are collapsed behind the first one).

NOTE

You can only build stacks with the contents of a folder (in the Folders
panel). If you click on a collection, or filter down photos using keywords,
metadata, or the Find panel, you won’t be able to stack them. You have to
go to the Folders panel and choose a folder of photos there (and yes, you can
only stack photos from the same folder).